1. Field of the Invention
This invention relates to a rotary compressor, and more particularly to a rotary compressor having two rotary compression mechanisms driven in common by a single rotating shaft supported by journal bearings.
2. Description of the Prior Art
As is well known, a refrigerator or an air conditioner requires a gas compressor. As a compressor for such a use, a rotary compressor is generally used because it can be readily made compact. A rotary compressor is usually constructed such that an electric motor and a compression mechanism driven by this motor are united within a single housing. The compression mechanism has a cylinder and a ring-shaped piston disposed eccentrically within the cylinder. A blade is attached to the cylinder so as to always make slidable contact with the outer circumference of the piston. The blade partitions the inside of the cylinder into a suction chamber and a compression chamber. The suction chamber has a gas-suction inlet, and the compression chamber has a gas-discharging outlet. The housing also serves as a tank to store gas compressed by the rotary compression mechanism.
A two-cylinder type rotary compressor has two rotary compression mechanisms which are driven by a single rotating shaft in common. The two-cylinder type rotary compressor has two rotary compression mechanisms disposed coaxially with respective blades that coincide in phase. The respective pistons of the two rotary compression mechanisms are securely fixed to the outer circumference of the rotating shaft with a phase difference of 180 degrees. Therefore, the two-cylinder type rotary compressor discharges compressed gas twice during one rotation of the rotating shaft. Thus, the two-cylinder type compressor has advantages in that torque fluctuations of the rotating shaft are smaller than in a one-cylinder type rotary compressor. As a result, smaller vibrations and lower noise can be achieved.
Recently, in the field of refrigerators and air conditioners, for the purpose of enhancement of operating efficiency and expansion of controllability, techniques of controlling a compressor with a variable speed control have been employed. The two-cylinder type rotary compressor incorporated in such appliances also has been required to achieve higher rotation performance. Improvement of the rotation performance of the two-cylinder type rotary compressor primarily requires a reduction in vibration and an improvement of relaibility of the bearing portions. For reduction in vibration, balancers are usually fixed at appropriate portions of the rotating shaft so as to compensate for dynamic imbalances of rotation.
However, it is difficult to completely eliminate dynamic imbalances of rotation. In addition, lateral load fluctuations act on the rotating shaft. Thus, the whirling of the rotating shaft is relatively large. This is the same even in the two-cylinder type rotary compressor.
A journal bearing which is superior in durability is usually used as a bearing for the rotary compressor. As is known, the journal bearing interposes an oil film between the journal of the rotating shaft and the inner surface of the journal bearing. The rotating shaft is supported against the oil film pressure. Thus, to exhibit a satisfactory bearing function, it is necessary to invariably introduce lubricating oil into the gap between the journal of the rotating shaft and the journal bearing. For this reason, an oil-guiding groove is formed extending axially on the outer circumferential surface of the rotating shaft, or on the inner surface of the journal bearing. As a result, the lubricating oil is introduced into the gap between the journal of the rotating shaft and the journal bearing by way of the oil-guiding groove.
However, when the above-described whirling of the rotating shaft arises, pressure variations occur in the gap between the journal of the rotating shaft and the journal bearing. Thus, it is difficult to invariably introduce the lubricating oil into the gap of the bearing. This causes the operational efficiency of the rotary compressor to decrease. Moreover, insufficient lubrication causes a direct contact between the bearing and the journal of the rotating shaft. Thus, the bearing and the rotating shaft are frequently damaged. In addition, adoption of the variable speed control technique allows high speed rotation of the rotating shaft. As is known centrifugal force caused by the eccentric rotations increases in proportion to the square of the number of revolutions. Thus, the load of the bearing, which is caused by the deflection of the rotation shaft, increases significantly. Therefore, the importance of appropriate lubrication, including a satisfactory oil-guiding groove has increased.
On the other hand, at present, noise from the two-cylinder type rotary compressor does not differ significantly from that of the one-cylinder type rotary compressor. Reduction in such noise is more difficult to achieve than a reduction in vibrations.
The characteristic noise from the two-cylinder type rotary compressor is a so-called beat, which is relatively noticeable. The beat is derived from the fact that a compressed gas is discharged by two pistons twice at intervals of 180 degrees per one revolution of the rotating shaft.
Specifically, in the case of the two-cylinder type rotary compressor, when the rotation frequency of the rotating shaft is defined as f.sub.s Hz, the above-described gas discharge operations produce a load fluctuation and a gas discharge pulsation of 2f.sub.s Hz. Thus, basically, a noise oscillation of 2f.sub.s Hz is generated.
Moreover, when the power source frequency of the motor is defined as f.sub.o Hz, the motor that drives the rotation shaft generates a magnetic oscillation of 2f.sub.o Hz due to magnetic unbalance.
Further, in the case of the two-cylinder type rotary compressor, unlike the one-cylinder type one, the above-described noise frequency of 2f.sub.s Hz is relatively large. Thus, the frequency difference between 2f.sub.o Hz and 2f.sub.s Hz is extremely small. Therefore, a beat of low frequency of 2(f.sub.o -f.sub.s)Hz is generated. The beat becomes a noticeably objectionable noise.